1. Field of the Invention
This invention relates to a radiation image read-out method and apparatus. This invention particularly relates to a radiation image read-out method and apparatus, wherein a radiation image having been stored on a stimulable phosphor sheet is read out with a line sensor comprising a plurality of photoelectric conversion devices, which are arrayed in a straight line.
2. Description of the Related Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems for computed radiography (CR). Specifically, radiation carrying image information of an object, such as a human body, is irradiated to a stimulable phosphor sheet, which comprises, for example, a substrate and a layer of the stimulable phosphor overlaid on the substrate. In this manner, a radiation image of the object is recorded on the stimulable phosphor sheet. Thereafter, with radiation image read-out apparatuses, stimulating rays, such as a laser beam, are irradiated to the stimulable phosphor sheet, on which the radiation image of the object has been stored. The stimulating rays cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet is photoelectrically detected and converted into an electric image signal.
Novel radiation image read-out apparatuses for use in the radiation image recording and reproducing systems utilizing the stimulable phosphor sheets described above have been proposed in, for example, Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540. In the proposed radiation image read-out apparatuses, from the point of view of keeping the emitted light detection time short, reducing the size of the apparatus, and keeping the cost low, a line light source for irradiating linear stimulating rays onto a stimulable phosphor sheet is utilized as a stimulating ray source, and a line sensor comprising a plurality of photoelectric conversion devices arrayed along the length direction of a linear area of the stimulable phosphor sheet, onto which linear area the stimulating rays are irradiated by the line light source, is utilized in order to detect the light emitted by the stimulable phosphor sheet. (The length direction of the linear area of the stimulable phosphor sheet will hereinbelow be referred to as the main scanning direction.) Also, the proposed radiation image read-out apparatuses comprise scanning means for moving the stimulable phosphor sheet with respect to the line light source and the line sensor and in a direction, which is approximately normal to the length direction of the linear area of the stimulable phosphor sheet. (The direction, which is approximately normal to the length direction of the linear area of the stimulable phosphor sheet, will hereinbelow be referred to as the sub-scanning direction.)
However, in the radiation image read-out apparatuses described above, when the linear stimulating rays are irradiated to the stimulable phosphor sheet, the light emitted from the exposed area of the stimulable phosphor sheet takes on the form of a spread light having predetermined spread (of approximately 600 xcexcm in the sub-scanning direction) due to scattering of the stimulating rays within the stimulable phosphor sheet and diffusion of the light emitted by the stimulable phosphor sheet. Also, in cases where a charge coupled device (CCD) is utilized as each of the photoelectric conversion devices for receiving the light emitted by the stimulable phosphor sheet, it is not always possible to set the width of the CCD, which width is taken in the sub-scanning direction, to be large. Practically, the width of the CCD, which width is taken in the sub-scanning direction, is capable of being set to be at most approximately 100 xcexcm. The width of the CCD, which width is taken in the sub-scanning direction, is thus smaller than the width of the spread light described above. Therefore, the CCD cannot receive all of the light emitted from the corresponding site of the exposed area of the stimulable phosphor sheet, and the light collecting efficiency cannot be kept high. Also, such that sufficient radiation absorption may be obtained, it is necessary for the stimulable phosphor layer of the stimulable phosphor sheet to have a certain value of thickness. Therefore, even though the width of the linear stimulating rays impinging upon the linear area of the stimulable phosphor sheet, which width is taken in the sub-scanning direction, is narrow, the scattering of the stimulating rays within the stimulable phosphor sheet and the diffusion of the light emitted by the stimulable phosphor sheet occur inevitably, and it is not always possible to set the width of the spread light described above to be smaller than the width of the CCD. Further, in cases where an image of the spread light described above is formed on the photoelectric conversion device by use of a contracting optical system comprising a lens, the light collecting efficiency cannot be kept high. Furthermore, a technique has been proposed, wherein a tapered fiber is located on the light receiving surface of the CCD, and the spread light described above is thereby contracted, such that the light collecting efficiency may not become low. However, the proposed technique has the problems in that the tapered fiber is expensive, and limitation is imposed upon an aperture ratio.
The primary object of the present invention is to provide a radiation image read-out method wherein, in cases where light emitted from an area of a stimulable phosphor sheet exposed to stimulating rays has predetermined spread, and a width of each of photoelectric conversion devices for receiving the emitted light, which width is taken in a sub-scanning direction, is smaller than the width of the spread light, which width is taken in the sub-scanning direction, an efficiency with which the emitted light is collected is capable of being enhanced, and an image signal having a high signal-to-noise ratio is capable of being obtained.
Another object of the present invention is to provide an apparatus for carrying out the radiation image read-out method.
The present invention provides a radiation image read-out method, comprising the steps of:
i) linearly irradiating stimulating rays onto an area of a surface of a stimulable phosphor sheet, on which a radiation image has been stored, with stimulating ray irradiating means, such that the stimulating rays are linear along a main scanning direction on the surface of the stimulable phosphor sheet, the stimulating rays causing the stimulable phosphor sheet to emit light in proportion to an amount of energy stored thereon during its exposure to radiation,
ii) receiving the light, which is emitted from the linear area of the surface of the stimulable phosphor sheet exposed to the linear stimulating rays, with a line sensor via a light collecting optical system, the line sensor comprising a plurality of photoelectric conversion devices arrayed in a straight line along the main scanning direction, the received light being subjected to photoelectric conversion performed by the line sensor,
iii) moving the stimulable phosphor sheet with respect to the stimulating ray irradiating means and the line sensor and in a sub-scanning direction, and
iv) successively reading outputs of the line sensor in accordance with the movement,
wherein an optical guide member is located between the light collecting optical system and the line sensor, the optical guide member having a reflection surface for reflecting the emitted light components of the emitted light, which emitted light components have been radiated out from the light collecting optical system and have spread over a range wider than a width of each of the photoelectric conversion devices, the width being taken in the sub-scanning direction, such that the photoelectric conversion devices are capable of receiving the emitted light components.
The present invention also provides a radiation image read-out apparatus for carrying out the radiation image read-out method in accordance with the present invention. Specifically, the present invention also provides a radiation image read-out apparatus, comprising:
i) stimulating ray irradiating means for linearly irradiating stimulating rays onto an area of a surface of a stimulable phosphor sheet, on which a radiation image has been stored, such that the stimulating rays are linear along a main scanning direction on the surface of the stimulable phosphor sheet, the stimulating rays causing the stimulable phosphor sheet to emit light in proportion to an amount of energy stored thereon during its exposure to radiation,
ii) a line sensor for receiving the light, which is emitted from the linear area of the surface of the stimulable phosphor sheet exposed to the linear stimulating rays, via a light collecting optical system, and performing photoelectric conversion of the received light, the line sensor comprising a plurality of photoelectric conversion devices arrayed in a straight line along the main scanning direction,
iii) scanning means for moving the stimulable phosphor sheet with respect to the stimulating ray irradiating means and the line sensor and in a sub-scanning direction, and
iv) reading means for successively reading outputs of the line sensor in accordance with the movement,
wherein an optical guide member is located between the light collecting optical system and the line sensor, the optical guide member having a reflection surface for reflecting the emitted light components of the emitted light, which emitted light components have been radiated out from the light collecting optical system and have spread over a range wider than a width of each of the photoelectric conversion devices, the width being taken in the sub-scanning direction, such that the photoelectric conversion devices are capable of receiving the emitted light components.
The term xe2x80x9cphotoelectric conversion devicexe2x80x9d as used herein means a CCD, a photodiode of a CMOS image sensor, or the like.
The expression of xe2x80x9cemitted light components having been radiated out from a light collecting optical system and having spread over a range wider than a width of each of photoelectric conversion devices, the width being taken in a sub-scanning directionxe2x80x9d as used herein means the emitted light components, which have spread in the sub-scanning direction due to scattering of the stimulating rays within the stimulable phosphor sheet and diffusion of the light emitted by the stimulable phosphor sheet, and all of which cannot be collected onto the light receiving surfaces of the photoelectric conversion devices of the line sensor with the light collection by the light collecting optical system alone.
In the radiation image read-out method and apparatus in accordance with the present invention, the reflection surface of the optical guide member need not necessarily be a flat surface and may be a curved surface.
Also, in the radiation image read-out method and apparatus in accordance with the present invention, the optical guide member should preferably have a refractive index such that the emitted light components are totally reflected by the reflection surface of the optical guide member. Alternatively, in cases where the refractive index of the optical guide member is not set such that the emitted light components are totally reflected by the reflection surface of the optical guide member, the reflection surface of the optical guide member may be covered with a reflecting material capable of reflecting the emitted light components.
Further, the radiation image read-out method and apparatus in accordance with the present invention should preferably be modified such that the optical guide member is a trapezoidal prism constituted of a transparent member, the trapezoidal prism being located close to a light receiving surface of the line sensor,
a surface of the trapezoidal prism, the surface having a rectangular shape whose one side is constituted of a longer side between two parallel sides of the trapezoid, acts as a light entry end face, from which the emitted light enters into the trapezoidal prism,
a surface of the trapezoidal prism opposite to the light entry end face acts as a light radiating end face, from which the emitted light is radiated out of the trapezoidal prism,
at least either one of two rectangular surfaces other than the light entry end face and the light radiating end face acts as the reflection surface,
whereby the emitted light components having entered from the light entry end face into the trapezoidal prism are reflected by the reflection surface and radiated out from the light radiating end face.
The term xe2x80x9ctrapezoidal prismxe2x80x9d as used herein means the prism having a trapezoidal cross-section.
Furthermore, the radiation image read-out method and apparatus in accordance with the present invention should preferably be modified such that the trapezoidal prism constituting the optical guide member is divided into a plurality of subdivisions at predetermined pitches with respect to the main scanning direction, and
a divisional cross-section of each of the subdivisions of the trapezoidal prism is set to be capable of reflecting the emitted light.
In such cases, the predetermined pitches, at which the trapezoidal prism constituting the optical guide member is divided into the plurality of the subdivisions, should preferably be identical with the array pitches of the photoelectric conversion devices with respect to the main scanning direction.
Also, in such cases, the divisional cross-section of each of the subdivisions of the trapezoidal prism should preferably be covered with a reflecting material capable of reflecting the emitted light.
The term xe2x80x9creflecting materialxe2x80x9d as used herein means one of various materials capable of reflecting the emitted light.
The term xe2x80x9ccovering with a reflecting materialxe2x80x9d as used herein means, for example, coating of the reflecting material on the reflection surface or the divisional cross-section of the optical guide member, formation of a layer of the reflecting material with a vacuum evaporation technique, or the like.
Also, the radiation image read-out method and apparatus in accordance with the present invention should preferably be modified such that the optical guide member is a prism, which is located on a side outward from at least either one of two ends of a light receiving surface of the line sensor, the two ends being taken with respect to the sub-scanning direction, and at a position close to the at least either one of the two ends of the light receiving surface of the line sensor, and which extends along the main scanning direction.
In such cases, the shape of the prism, which is located on the side outward from the at least either one of the two ends of the light receiving surface of the line sensor, maybe selected from various shapes, such as a triangular prism and a trapezoidal prism, which have the reflection surfaces described above. However, the prism, which is located on the side outward from the at least either one of the two ends of the light receiving surface of the line sensor, should preferably have a shape such that the prism is capable of being located easily on the side outward from the at least either one of the two ends of the light receiving surface of the line sensor, the two ends being taken with respect to the sub-scanning direction, and at the position close to the at least either one of the two ends of the light receiving surface of the line sensor.
As will be understood from the specification, it should be noted that the term xe2x80x9cmoving a stimulable phosphor sheet with respect to stimulating ray irradiating means and a line sensor and in a sub-scanning directionxe2x80x9d as used herein means movement of the stimulable phosphor sheet relative to the stimulating ray irradiating means and the line sensor, and embraces the cases wherein the stimulable phosphor sheet is moved while the stimulating ray irradiating means and the line sensor are kept stationary, the cases wherein the stimulating ray irradiating means and the line sensor are moved while the stimulable phosphor sheet is kept stationary, and the cases wherein both the stimulable phosphor sheet and the stimulating ray irradiating means and the line sensor are moved. In cases where the stimulating ray irradiating means and the line sensor are moved, they should be moved together with each other.
With the radiation image read-out method and apparatus in accordance with the present invention, the emitted light components of the emitted light, which emitted light components have been radiated out from the light collecting optical system and have spread over the range wider than the width of each of the photoelectric conversion devices, the width being taken in the sub-scanning direction, are reflected by the reflection surface of the optical guide member and received by the photoelectric conversion devices. Therefore, the efficiency with which the emitted light is collected is capable of enhanced, and an image signal having a high signal-to-noise ratio is capable of being obtained.
With the radiation image read-out method and apparatus in accordance with the present invention, the optical guide member may have the refractive index such that the emitted light components are totally reflected by the reflection surface of the optical guide member. In such cases, a process for covering the reflection surface with the reflecting material, or the like, need not be performed, and the effects described above are capable of being obtained.
With the radiation image read-out method and apparatus in accordance with the present invention, the optical guide member may be the trapezoidal prism constituted of the transparent member, and the trapezoidal prism may be located close to the light receiving surface of the line sensor. In such cases, the optical guide member is capable of being produced easily.
With the radiation image read-out method and apparatus in accordance with the present invention, the trapezoidal prism constituting the optical guide member may be divided into the plurality of the subdivisions at predetermined pitches with respect to the main scanning direction. Also, the divisional cross-section of each of the subdivisions of the trapezoidal prism may be set to be capable of reflecting the emitted light. In such cases, the problems are capable of being prevented from occurring in that the light emitted from a certain site (a certain pixel) on the stimulable phosphor sheet is adversely affected by the light emitted from a pixel on the stimulable phosphor sheet, which pixel is adjacent to the certain pixel in the main scanning direction. Therefore, an image signal representing an image having a high image sharpness is capable of being obtained.
With the radiation image read-out method and apparatus in accordance with the present invention, the optical guide member may be the prism, which is located on the side outward from the at least either one of the two ends of the light receiving surface of the line sensor, the two ends being taken with respect to the sub-scanning direction, and at the position close to the at least either one of the two ends of the light receiving surface of the line sensor, and which extends along the main scanning direction. In such cases, the optical guide member is capable of being easily fitted to the line sensor.
With the radiation image read-out method and apparatus in accordance with the present invention, wherein the reflection surface of the optical guide member is covered with the reflecting material capable of reflecting the emitted light components, a material having a high refractive index need not be utilized as the material of the optical guide member, and the emitted light components are capable of being reflected efficiently.